Choosing a system, the practical and the philosophical

I come from a background of an old Pentax K mount SLR film camera and a long
period away from photography. A couple of years ago I bought a cheap
super-zoom digital fixed lens just to find out what the digital revolution
was about. I am now frustrated with its limitations and looking to go DSLR.
I don't have a lot of money but enough to get started. I like to photograph
the natural world: eagles, landscapes, insects and flowers. I can see
the budget will have to cover several lenses eventually (sigh).

Given the price of lenses once you start with a system (eg Nikon or Canon)
you tend to stay with it, I don't see that many are going to jump from one
to the other although I suppose its possible. This explains why people
stick to a system but not why they selected it in the first place. I know
there are other systems but for the point of discussion let's stick to those
two. Why choose one over the other? A couple of possibilities come to
mind, no doubt there are plenty that I haven't thought of.

One is that the buyer was attracted to a particular body at a point in time
and bought lenses to go with it. This suggests that at some other point in
time they could have gone with the opposition if they had a body in their
line-up that attracted the buyer more. This implies that there is no
intrinsic difference between the competitors but that over time their
systems leapfrog each other in appeal according to the models in the
catalog.

Another is that there is some intrinsic difference between the systems. As
neither seem to be fading into oblivion if such a difference exists it seems
to be one of style or approach not of basic suitability for purpose. Is
there such a difference? If so what is? What kind of photographer is
attracted to one or the other?

I suppose a third is that they were given a Nikon or that Daddy always used
Canon and that is what they learned on, that is the photographer didn't
really choose but fell into it. I have no such initial conditions.

There could be other reasons for choosing one system over another. What?

Is this issue covered on the WWW or in any literature? Where?

I am after such general advice that comes from experience and not from sales
brochures. If you recommend one or the other I am more interested in the
reason why than the recommendation itself, as I might have different needs
and abilities to yours. I am not trying to start a flame war, I have no
axe to grind nor (I hope) any preconceived ideas.

David